This invention relates to a method of evaluating the surface of a solid body, a method of evaluating a magnetic disk, a magnetic disk, and a manufacturing method thereof. More specifically, this invention relates to a magnetic disk that is reduced in surface energy to enable prevention of fly stiction in a flying height of a magnetic recording head being 10 nm or less, and a manufacturing method of the magnetic disk.
In various industrial fields, “wettability” of solid body surfaces is cited as an important concept.
Particularly, the property of repelling “water”, i.e. “water repellency”, is utilized in design and manufacture of all industrial products such as design of high molecular materials, surface treatment to paper and fiber surfaces, water repellent finish of glass surfaces, printing, and lubricant design for hard disk surfaces.
When a liquid contacts the surface of a solid body so as to be hemispheric and forms a contact angle, this contact angle is determined by equilibrium among surface tension of the liquid, surface energy of the solid body, and solid-liquid interfacial energy. Use is often made of a method that utilizes this principle and calculates a surface energy of a solid body surface from a contact angle of a liquid whose surface tension is known.
In general, as a standard of surface free energy of a solid body surface, use is made of a critical surface tension (γc) of Zisman (e.g. see Document 1 “J. Phys. Chem.”, vol. 58, pp. 236 (1954)). According to Zisman et al., the critical surface tension (γc) is defined as follows. When contact angles of a solid body surface are measured with respect to a series of saturated hydrocarbon liquids having different surface tensions and cosines (cos θ) of the contact angles are plotted for the surface tensions, a straight line is formed. In this straight line, a value of surface tension at a point extrapolated to a point where the cosine (cos θ) of the contact angle becomes 1 (i.e. a point that is completely wet) is the critical surface tension (γc).
As values of γc in documents, those about fluorine compounds are low. Among them, a perfluorolauric acid monomolecular film having —CF3 oriented on its surface, which is described in the foregoing Document 1 published in 1954 by Zisman et al., is often cited even now as the one having the lowest value (6 mN/m). It is noted here that mN/m represents milli Newton/meter.
The foregoing critical surface tension (γc) defined by Zisman et al. has a certain effectiveness even at present, but there are problems shown below.
(1) A relationship between surface tensions of liquids and cos θ does not become a linear relationship particularly when the surface tensions of the liquids become large.
(2) Depending on the presence of a hydrogen bonding property, polarity, and nonpolarity possessed by liquids, plotted points are divided into groups and do not form one straight line.
In case where surface energy of a solid body surface is evaluated, it cannot be said that a method of deriving a critical surface tension only by nonpolar saturated hydrocarbon liquids defined by Zisman et al. shows the true surface energy because the influence of other polarity components and hydrogen bond components is ignored.
Further, when a contact angle of water is inversely calculated by the use of an approximate expression of linear extrapolation that derives a critical surface tension of a perfluorolauric acid monomolecular film using a series of saturated hydrocarbon liquids, cos θ=−1.47 is obtained, which obviously deviates from the natural laws.
On the other hand, at present, drastic technological innovation is required in information recording technique, particularly in magnetic recording technique following the development of the IT industry. In a magnetic disk installed in a magnetic disk device such as an HDD (hard disk drive), a technique that can achieve an information recording density of 100 Gbit/inch2 or more is required. In this magnetic disk, conventionally, a magnetic layer serving for information recording is provided on a substrate and, on the magnetic layer, a protection layer for protecting the magnetic layer and a lubrication layer that relaxes interference from a flying magnetic head are provided.
In the demand for higher recording densities in recent years, various approaches have been attempted in order to achieve the information recording density of 100 Gbit/inch2 or more. As one of them, in order to improve spacing loss to thereby increase the S/N ratio, it is required to narrow a gap (magnetic spacing) between a magnetic layer of a magnetic disk and a recording/reproducing element of a magnetic head to 20 nm or less. From the viewpoint of achieving this magnetic spacing, it is required that the thickness of a protection layer of the magnetic disk be reduced to 5 nm or less. It is further required that the flying height of the magnetic head is reduced to 10 nm or less. Moreover, it is required that an LUL system (load/unload system) capable of higher capacity is used as a start/stop mechanism of an HDD instead of the conventional CSS system.
Here, in the LUL system, upon stopping, a magnetic recording head is retreated to a slope bed called a ramp located outside a magnetic disk while, upon starting, the magnetic recording head is, after the magnetic disk starts rotation, slided from the ramp over the surface of the magnetic disk, and then recording/reproduction is carried out. Accordingly, the magnetic recording head does not contact and slide on the magnetic disk.
In this LUL system, it is not necessary to provide a contact/slide region (CSS zone) for the magnetic recording head on the surface of the magnetic disk, which is required in the CSS system conventionally used. Therefore, it is possible to ensure a wider area of a recording/reproduction region as compared with the CSS system and thus increase the recording capacity of the magnetic recording medium. Further, in the LUL system, since the magnetic disk and the magnetic recording head do not contact each other, it is not necessary provide texture for preventing contact adsorption, which is required in the CSS system, so that the surface of the magnetic disk can be made even smoother. Therefore, the recording density of the magnetic disk can be increased by reducing the flying height of the magnetic recording head as compared with the CSS system.
In particular, recently, a magnetic disk apparatuses such as HDDs (hard disk drives) are often used in environments of low atmospheric pressures such as in an airplane. Following this, there has been arising a problem about flying stability of magnetic heads. Specifically, the flying height of the magnetic head further decreases from 10 nm due to a change in atmospheric pressure and, further, there is variation in flying height due to processing accuracy of air bearing sliders of the magnetic heads. As a result, a problem of fly stiction has occurred frequently. The fly stiction is a trouble where the flying posture and height of the magnetic recording head go out of order during its flying operation and irregular changes in reproduction output occur frequently. According to circumstances, the magnetic recording head is brought into contact with the magnetic disk so as to crash during the flying operation, thereby destroying the magnetic disk. This fly stiction often occurs without a premonitory sign and is one of troubles that are difficult to control. As a cause for occurrence of the fly stiction, there can be considered the influence of roughness of the surface of the magnetic disk, interaction (meniscus force) between a lubrication layer and the head, or contamination due to outgas from the magnetic disk device.
According to the study by the present inventors, it has been found that the foregoing fly stiction phenomenon has a close relationship with surface energy of the magnetic disk surface. Specifically, the mechanism for preventing the fly stiction phenomenon is as follows. By reducing the surface energy of the magnetic disk surface, the magnetic disk surface changes to a more inactive state. Because of the inactive surface, the interaction with the magnetic recording head becomes small and further the outgas from the magnetic disk apparatus cannot easily adhere to the magnetic disk surface so that the flying stability of the magnetic recording head increases.